Abstract:
The economic viability of CO2 reactors is contingent on the selectivity of the CO2 reduction reaction and the rate of product formation. For this, the rational design of electrolyzers also has a substantial impact on the figures of merit (current density, faradaic efficiency, cell durability). Thus, herein we portray a short review on the shortcomings, challenges and the recent developments on different reactor configurations, components and membrane structures for the efficient electrochemical CO2 reduction (CO2R) into HCOO-/HCOOH. Despite their low CO2 solubility and poor mass transport, H-type electrolyzers are commercialized due to their screening of a vast number of catalysts. In contrast, membrane-based gas and liquid phase flow reactors break the barriers faced by H-types through the incorporation of gas diffusion electrodes (GDEs) and the membrane electrode assembly (MEA). As the GDE forms the gas-liquid-solid interface, it allows the electrolyzers to generate current densities at the industrial level (200 mA cm(-2)). Intriguingly, a continuous liquid fed intermittent flow electrolyzer can control the electrolyte flow at a desired frequency and allow sufficient time for CO2 gas molecules to effectively reduce into HCOOH. Therefore, a high and stable faradaic efficiency (95%) is achieved in 4 h for HCOOH (576.98 mg) using the boron-doped diamond catalyst. Very recently, a novel strategy to enhance the CO2R to HCOO-/HCOOH has been adopted via the recirculation of by-products to the liquid phase MEA flow reactors, which substantially improves HCOO- selectivity, lowers material costs, and promotes CO2 mass transfer. In the end, the zero-gap electrolyzer has newly emerged and affords reduced ohmic losses, leading to a straight-forward implementation of industrial systems for CO2R to value-added products in the future. Besides, the efficiency of HCOO-/HCOOH production is also explored against proton exchange, anion exchange and bipolar membranes, and the pH of the electrolyte plays a dominant role in deciding the stability and characteristics of the membranes. It is also depicted that the product selectivity depends on different electrolyzer configurations. Recently, bimetallic alloys (Bi-Sn, Bi-In) and 2D layered composites (SnO2/rGO/CNT) have proven to be potential electrocatalysts (faradaic efficiency > 95%, highly selective and durable) assigned to the abundant active sites for CO2R. Based on the recent findings and future research directions, we draw reader's attention to construct economic, scalable and energy-efficient CO2R electrolyzers to realize the techno-economic predictions.